The phenomenon of bioluminescence was observed in some living species, and was based on the chemical reaction of a luciferin (a luminescence substrate) and a luciferase (an enzyme catalyzes the luminescence reaction) in vivo. A number of researches including the researches for identification of luciferin and luciferase and for elucidation of the luminesence mechanism in a molecular level have been performed inside and outside of Japan.
In recent years, bioluminescence is used as a tool for biological research. In addition, the applied researches in the medical field including high through-put screening (HTS) of drugs, intramolecular imaging, etc., have been intensively developed on the basis of the principle of bioluminescence.
Fireflies, sea pansies Renilla, sea fireflies Cypridina, deep-sea shrimps Oplophorus, luminescent microorganisms, etc. are known as representative bioluminescent organisms that produce bioluminescence. The jellyfish Aequorea victoria is also a bioluminescent animal, but the bioluminescence of the jellyfish is not produced by the luciferase reaction. The luminescence is produced by the Ca2+-triggered reaction of the photoprotein of aequorin, the complex of substrate-enzyme-molecular oxygen. It is known that many organisms utilize the compound having an imidazopyrazinone skeleton as a luminescence substrate in the bioluminescence system.
Among them, coelenterazine (CTZ) is a compound commonly used as a luminescence substrate (luciferin) for aequorin which is a photoprotein from jellyfish, or for luciferases from some bioluminescent organisms such as sea pansies Renilla, etc. Therefore, many findings of CTZ have been accumulated.

In fact, approximately 50 types of coelenterazine analog (CTZ analog) have been synthesized heretofore, and the substrate specificity for some of them has been examined in several bioluminescence systems (cf., e.g., Non-Patent Literatures 1 to 5).